FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 9. Aircraft Systems and Components | 2. Hydraulic Systems | 9-29. Hydraulic System Maintenance Practices
9/8/98
AC 43.13-1B
technician refer to the troubleshooting infor
mation furnished by the manufacturer.
(1) Lack of pressure in a system can be
caused by a sheared pump shaft, defective re
lief valve, the pressure regulator, an unloading
valve stuck in the “kicked-out” position, lack
of fluid in the system, the check valve installed
backward, or any condition that permits free
flow back to the reservoir or overboard. If a
system operates satisfactorily with a ground
test unit but not with the system pump, the
pump should be examined.
(2) If a system fails to hold pressure in
the pressure section, the likely cause is the
pressure regulator, an unloading valve, a leak
ing relief valve, or a leaking check valve.
(3) If the pump fails to keep pressure up
during operation of the subsystem, the pump
may be worn or one of the pressure-control
units may be leaking.
(4) High pressure in a system may be
caused by a defective or improperly-adjusted
pressure regulator, an unloading valve, or by
an obstruction in a line or control unit.
(5) Unusual noise in a hydraulic system,
such as banging and chattering, may be caused
by air or contamination in the system. Such
noises can also be caused by a faulty pressure
regulator, another pressure-control unit, or a
lack of proper accumulator action.
(6) Maintenance of hydraulic system
components involves a number of standard
practices together with specialized procedures
set forth by manufacturers such as the re
placement of valves, actuators, and other
units, including tubing and hoses. Care should
be exercised to prevent system contamination
damage to seals, packings, and other parts, and
to apply proper torque in connecting fittings.
When installing fittings, valves, etc. always
lubricate the threads with hydraulic fluid.
(7) Overhaul of hydraulic and pneu
matic units is usually accomplished in ap
proved repair facilities; however, replacement
of seals and packings may be done from time
to time by technicians in the field. When a
unit is disassembled, all O-ring and Chevron
seals should be removed and replaced with
new seals. The new seals must be of the same
material as the original and must carry the cor
rect manufacturer’s part number. No seal
should be installed unless it is positively iden
tified as the correct part and the shelf life has
not expired.
(8) When installing seals, care should
be exercised to ensure that the seal is not
scratched, cut, or otherwise damaged. When it
is necessary to install a seal over sharp edges,
the edges must be covered with shim stock,
plastic sheet, or electrical tape.
(9) The replacement of hydraulic units
and tubing usually involves the spillage of
some hydraulic fluid. Care should be taken to
ensure that the spillage of fluid is kept to a
minimum by closing valves, if available, and
by plugging lines immediately after they are
disconnected. All openings in hydraulic sys
tems should be capped or plugged to prevent
contamination of the system.
(10) The importance of the proper
torque applied to all nuts and fittings in a sys
tem cannot be over-emphasized. Too much
torque will damage metal and seals, and too
little torque will result in leaks and loose parts.
The proper torque wrenches with the appropri
ate range should be used in assembling system
units.
f. Disposal of Used Hydraulic Fluids. In
the absence of organizational guidelines, the
Par 9-29
Page 9-17